Process for the stabilization of acrylonitrile by distillation with a nitroxide

ABSTRACT

WHEREIN R1, R2, R3, and R4 are alkyl groups and no hydrogen is bound to the remaining valancies on the carbon atoms bound to nitrogen. A preferred nitroxide is 2,2,6,6 - tetramethyl piperid-4-one-1 oxyl which may be used in the form of its clathrate compound with straight chain hydrocarbons.   A process for the recovery of acrylonitrile by distillation by adding to the distillation feedstock a nitroxide having the essential skeletal structure:

United States Patent 1 Bailey July 24,1973

[75] Inventor: Hubert Charles Bailey, London,

England 731 Assignee: BP Chemicals Limited, London,

England 22 Filed: 0ct.4, 1971 21 Appl. No.: 186,469

[30] Foreign Application Priority Data Oct.-5, 1970 Great Britain47,169/70 [52] US. Cl .L 203/8, 203/57, 203/58, 260/4659 [51] Int. Cl.C07c 121/32 [58] Field of Search 203/8, 9, 57, 58; 260/4659 [56] IReferences Cited UNITED STATES PATENTS 8/1956 Couvillon 260/4659Sennewald et a1 260/4659 Nakajima et al 260/4659 Primary Examiner-WilburL. Bascomb, Jr. AttorneyBrocks, Haidt & Haffner [57] ABSTRACT A processfor the recovery of acrylonitrile by distillation by adding to thedistillation feedstock a nitroxide having the essentialskeletal'structure:

wherein R,, R R and R are alkyl groups and no hydrogen is bound to theremaining valancieson the carbon atoms bound to nitrogen. A preferrednitr'oxide is 2,2,6,6 tetramethyl piperid-4-one-l -oxyl which may beused in the form of its clathrate compound with straight chainhydrocarbons.

10 Claims, N0 Drawings PROCESS FOR THE STABILIZATION OF ACRYLONITRILE BYDISTILLATION WITH A NITROXIDE This invention relates to a process forthe stabilisation of acrylonitrile.

Processes for the production of acrylonitrile by the vapour phasereaction of propylene or acrolein with ammonia and molecular oxygen inthe presence of a solid catalyst are well known. In the working up ofthe acrylonitrile product it has been found that the acrylonitrile tendsto polymerise especially under distillation conditions even whenconventional inhibitors e.g., hydroquinone, oxygen etc., are present.Polymer deposits form in the distillation columns and in theinterconnecting lines, particularly in dead-spaces or vapour-lines,which are not continuously washed with condensate. These depositsinterfere with the efficient operation of the plant and eventually buildup to such an extent that the plant must be shut down for cleaning. Shutdowns can occur at frequent intervals and the resulting loss ofproduction can be considerable.

The polymer is produced by free radical chain polymerisation of theacrylonitrile. In the presence of oxygen, peroxy radicals are formedwhich lead to the formation of acrylonitrile poly-peroxides. These areunstable and break down to radical species which thus autocatalyse, theoxidation. When the oxygen present in the system is consumed theseradical species derived from the polyperoxide can then initiatehomopolymerisation of the acrylonitrile. Thus when small traces ofoxygen are present in a distillation column, and it is very difficult toexclude oxygen entirely, the net result may be the formation of smallamounts of polyperoxide which can then initiate a considerably largerquantity of homopolymerisation. Though it is considered that a majorpart of the polymerisation occurring in distillation columns may beinitiated by such trace amounts of oxygen the successful application ofthe invention disclosed here does not depend on the mode of initiationof the free radical chains.

We have discovered that certain nitroxides are particularly suitablestabilisers for acrylonitrile under distillation conditions. Theyinhibit polymerisation at elevated temperatures and actin the absence ofoxygen although they can also tolerate the presence of oxygen and do notgive undesirable compounds when the latter is present.

It will be appreciated that the ability of the nitroxide to functionboth in the presence and absence of oxygen is a marked improvement overthe prior art in which a choice has to be made between an oxygen-freeand an oxygen-containing system, and in which any change from one systemto the other would lead to highly undesirable results.

Stable radicals of the formula:

(J42 \Hlz CH CH CH3 N CH3 is not obvious that the nitroxides of thepresent application would be particularly effective as polymerisationinhibitors at relatively high temperatures (60C) and above), both in thepresence and absence of water and oxygen nor that they would possessvolatilities sufficient to confer inhibition to the vapour and to thedead-spaces in the columns.

Thus according to the present invention there is provided a process forthe recovery of acrylonitrile by distillation, which process comprisesadding to the distillation feedstock a nitroxide having the essentialskeletal structure:

in R. (I)

wherein R,, R R and R, are alkyl groups and no hydrogen is bound to theremaining valencies on the carbon atoms bound to nitrogen.

The alkyl groups R, to R, may be the same or differem, and preferablycontain 1 to 15 carbon atoms. It is particularly preferred to usenitroxides having the essential skeletal structure (1) in which R, to R,are methyl, ethyl or propyl groups.

The remaining valencies of the carbon atoms in the essential skeletalstructure which are not satisfied by R, R or nitrogen may be satisfiedby any atom or group except hydrogen which can bond covalently to carbonalthough some groups may reduce the stabilizing power of the nitroxidestructure (1) and are undesirable. Examples of suitable atoms or groupsare halogen, cyannide when R is alkyl or aryl,

SC,,H,, -S-COCH,,, OCOCH -OCOC H, alkenyl where the double bond is notin conjugation with the group v R1 R3 the groups R and R preferablycontain 1 to carbon atoms. Examples of suitable groups R and R aremethyl, ethyl and propyl groups. A specific example of a suitablecompound having the essential structure (ll) is di-tert-butyl nitroxide.

Although as we have indicated above it is possible to stabilizeacrylonitrile in the presence of oxygen, for the very best results it ispreferred to exclude oxygen and oxygen-yielding materials. In thepresence of oxygen, peroxy compounds of acrylonitrile tend to be formed,which on decomposition lead to the consumption of nitroxide. However,the compositions of the present invention will still be stabilizedagainst polymerization in oxygen concentrations in the acrylonitrile ofup to those present in a monomer in equilibrium with air at atmosphericpressure. This is extremely useful as there is therefore no need topurge the acrylonitrile with inert gas to remove dissolved oxygen. Thisis in marked contrast to monomers stabilized with some other inhibitorseffective in the absence of oxygen. Thus nitric oxide may be used toinhibit the polymerisation of monomers in the absence of oxygen butwhere oxygen is present the inhibiting action of nitric oxide is lost.

Suitably the nitroxide is employed in a concentration in the range 1 to1000 parts per million parts of acrylonitrile.

The nitroxides are suitable for use over a wide range of temperatures,but typically in distillation columns the temperature is preferably inthe range 60 to 120C.

A preferred nitroxide, 2,2,6,6-tetramethyl-piperid- 4-one-l-oxyl, may beused in the form of its clathrate compounds with straight chainhydrocarbons e.g. the 2,2,6,6-tetramethyl-piperid-4-one-l-oxylderivative of C1040 n-paraffins.

The nitroxides of the present invention are particularly suitable foracrylonitrile stabilisation, because they are sufficiently volatile toconfer inhibition to the vapour phase, as encountered in a distillationcolumn, without, at the same time, contaminating the acrylonitriledistillate to an undesirable extent.

It is generally desirable to be able to remove inhibitor from theacrylonitrile, particularly if the acrylonitrile is to be used as amonomer in a polymerisation process. in this context a furthersurprising but useful feature of the present invention is that, ifdesired, the nitroxide inhibitors may be removed from the acrylonitrileby treatment with an acidic ion-exchange resin, for example amberlyst.

The nitroxides may be prepared by the method disclosed by O.L. Lebedevand S.N. Kazarnovski, Tr. po Khim. i Khim. Tekhnol. 2, 649-656 (1959).

The invention is illustrated by the following examples:

EXAMPLE 1 in an accelerated inhibition test, pure acrylonitrilecontaining as polymerisation initiator l,l-azobis (cyclohexane nitrile)(2.5 X 10" mole/ l) and as inhibitor,2,2,6,6-tetramethyl-4-hydroxy-piperidine-l-oxyl (2 x i0 mole/I), washeated at 100C in a sealed tube. The polymerisation induction period was90 minutes. From the known rate of formation of radicals from theinitiator, it was calculated that the radical terminating efficiency ofthe nitroxide was 98 percent.

Comparison Test 1 The procedure of Example 1 was repeated except that nonitroxide was added. The inhibition period was only 3-4 minutes.

The results of this test compared with those of Example 1 demonstratethe efficienty of 2,2,6,6-tetramethyl- 4-hydroxypiperidine -l-oxyl as apolymerisation inhibitor for acrylonitrile.

EXAMPLE 2 This example demonstrates the use of a clathrate as inhibitor.An accelerated test, carried out as in Example l, but with then-hexa'decane clathrate of 2,2,6,6- tetramethyl piperid-4-one-l-oxyl (2X 10 mole/l) as inhibitor gave an induction period of minutes. Theefficiency of radical termination by the nitroxide was perent.

EXAMPLE 3 In a simulation of a portion of a distillation column,acrylonitrile containing the nitroxide inhibitor was heated in areboiler under reflux, and a small sample tube of unstabilised pureacrylonitrile was suspended in the vapour-space. The sample tube wasopen at the upper end to allow contact with the surrounding vapour, butentry of the liquid reflux was prevented. A slow bleed of air was addedto the reboiler, below the liquid level. The rate of addition of air wasapproximately one one-hundred and fiftieth of the boil-'uprate of theacrylonitrile.

With SO ppm of the hydroxy-nitroxide in the reboiler, no polymer wasformed in the tube of unstabilised acrylonitrile after intermittentoperation over 6 weeks.

Comparison Test 2 The procedure of Example 3 was repeated except that inone test there was no inhibitor, and a second test, hydroquinon e (50ppm) was added to the boiler. The onset of polymerisation in both sampletubes was evident after only 4 to 5 days.

The results of Example 3 compared with those of Comparison Test 2demonstrate the effectiveness of nitroxides as polymerisation inhibitorsin the vapour phase.

I claim:

l. A process for the recovery of acrylonitrile by distillation whichprocess comprises adding to the distillation feedstock a nitroxideradical having the essential structure wherein each of R R R and R isalkyl containing 1 to carbon atoms and X and X is the same or differentand is alkyl, halogen, cyanide,

, wherein R is either alkyl or aryl,

NHz

, or X and X taken together form part of a ring.

. troxide 2. A process as claimed in claim 1 wherein the groups R to Rare alkyl groups selected from methyl, ethyl and propyl groups.

3. A process according to claim 1 wherein each of X and X is methyl,ethyl or propyl.

4. A process according to claim 1 wherein the nitroxide radical of saidstructure is di-tert.-butyl nitroxide.

5. A process according to claim 1, wherein X and X together form part ofa ring.

6. A process according to claim 5, wherein said nitroxide radical isselected from the group consistingof piperdinel -oxyls and pyrrolidinel-oxyls.

7. A process according to claim 6 wherein said niradical is2,2,6,6-tetramethyl-4-hydroxypiperidine- 1 -oxyl.

8. A process according to claim 6 wherein said nitroxide radical is2,2,6,6-tetramethyl-4-one-l-oxyl.

9. A process according to claim 8 wherein said 2,2,6,-6-tetramethyl-piperid-4-one-l-oiryl is in the form of its 3 UNETEDSTATES PATENT: ()FFEGM CERTEFKCATE 9F (Be Patent No. 3 ,747,988 Dated Jy" 1973 HUBERT CHARLES. BAILEY Inventofls) ppears' in theaboveeidentified patent;

It is certified that error a hereby corrected as shown below:

and that said Letters Patent ere Col 4, line 31 correct the spelling of"per cent". Original claim 10 was' omitted. Said claim 10 should: beprinted to read as follows: I

lO. process as claimed in Claim 1 wherein the nitroxide is employed at aconservtration in tghe range of l to 100E parts per a million parts of'acrylonitrile and th e distillation temperature is in the range 60? tol20C.'-'

Signed and sealed this 20th day of November 1973 (SEAL) Attest 2 EDWARD1\I.FLETCHER,JR. RENE D. TEGTIVIEYER Atte'sting Officer ActingCommissioner of Fate

2. A process as claimed in claim 1 wherein the groups R1 to R4 are alkylgroups selected from methyl, ethyl and propyl groups.
 3. A processaccording to claim 1 wherein each of X and X1 is methyl, ethyl orpropyl.
 4. A process according to claim 1 wherein the nitroxide radicalof said structure is di-tert.-butyl nitroxide.
 5. A process according toclaim 1, wherein X and X1 together form part of a ring.
 6. A processaccording to claim 5, wherein said nitroxide radical is selected fromthe group consisting of piperdine-1-oxyls and pyrrolidine-1-oxyls.
 7. Aprocess according to claim 6 wherein said nitroxide radical is2,2,6,6-tetramethyl-4-hydroxy-piperidine-1-oxyl.
 8. A process accordingto claim 6 wherein said nitroxide radical is2,2,6,6-tetramethyl-4-one-1-oxyl.
 9. A process according to claim 8wherein said 2,2,6,6-tetramethyl-piperid-4-one-1-oxyl is in the form ofits clathrate compounds with straight chain hydrocarbons selected fromC10 -C20 -n-paraffins.
 10. A process as claimed in claim 1 wherein thenitroxide is employed at a concentration in the range 1 to 1000 partsper million parts of acrylonitrile and the distillation temperature isin the range 60* to 120*C.